1. Field of Invention
This invention relates to a fluid-pressure powered actuator of small diameter producing large thrust, used to operate valves, brakes, and various articulated mechanisms.
2. Prior Art
Actuators powered by pneumatic or hydraulic pressure are the preferred means of remotely operating mechanical devices where the possibility of electrical shock, spark, or electromagnetic interference is too high.
When linear thrust is needed to operate some device, such as the poppet in a valve, actuators are used which vary in construction and complexity, ranging from a single piston in a closed cylinder directly coupled with an output member to more elaborate arrangements of levers and cams coupling a piston or bladder with an output member. Minimizing the cost usually argues for the simplest, single piston actuators, but occasions arise when the thrust needed to operate the device is greater and the space available for an actuator is restricted.
The thrust produced in a single piston is the product of its area projected in the direction of its linear motion and the pressure of the fluid, so when greater thrust is needed from a simple piston actuator, either the piston diameter or fluid pressure must be increased. Delivering greater fluid pressure to the actuator is not always a preferred option because it requires a stronger pump to produce it and heavier conduits to deliver it; greater pressure also carries greater risk of injury or damage from accidental fluid releases. When room is limited or has high overhead cost, as in clean rooms and other tightly controlled environments, increasing the piston diameter is also not preferred and other ways are sought to increase the actuator's output thrust.
One simple option is to gang together two or more pistons to increase the effective piston area; for practical considerations, this type uses three pistons in series at most and, because of effective area lost to the rods which connect the pistons, the resultant output force is less than 2.85 times that of a single piston actuator or the same diameter.
Another option to increase output thrust is an “oil-filled” actuator which uses a primary piston to drive a much smaller piston which pushes against a fixed volume of incompressible fluid, typically a hydraulic oil, which in turn pushes against a piston similar in diameter to the first piston; this arrangement essentially produces an internal fluid pressure much higher than that delivered to the actuator, resulting in a higher output thrust. Unlike actuators where the output member is directly coupled with the pistons driven by the remotely delivered fluid, “oil-filled” actuators have output member movements only a fraction of the first piston's movement, requiring longer overall actuator assembly length for a given output movement requirement, but this is seldom an important issue. These can provide very high actuation force without increasing the basic actuator diameter, but, because the higher internal fluid pressure increases the chance of gasket failure, this type of actuator carries a contamination or fire risk from leakage of its internally held fluid and is not preferred in environments such as semiconductor fabrication or pharmaceutical production facilities. This type of actuator can self-actuate due to thermal expansion of the internally held fluid with increasing temperatures if the actuator's assembly does not leave the first piston adequate back clearance to accommodate the expansion, or may act sluggishly at low temperatures, for which reason it is not preferred when operating temperatures can fluctuate widely.
Another option is a linear actuator which amplifies the thrust of its piston with sets of rollers against cam levers or more elaborate linkages, increasing the output thrust without appreciable increase in piston size, fluid pressure, or use of internally held fluids. These kinds of actuators have a minimum of five or six moving parts, compared with one or two in a simple piston actuator or two or three in a fluid-filled actuator, making them more troublesome to assemble and maintain. Scaling these designs down to smaller sizes (4 cm in diameter or less) is difficult because the areas of concentrated bearing and shear stress mean in these moving parts prevents them from being scaled down proportionally. These areas of concentrated stress are also subject to greater rates of wear, and these kinds of actuators are not preferred where maintenance actions carry a high overhead cost.
The oldest and simplest method of producing high linear motion and force in a relatively compact machine is with the power screw, in which an axially constrained nut with female threads is rotated to produce axial thrust in a rotationally constrained rod engaging it with male threads, or visa versa. This mechanism has been and is still used to operate every manner of device, especially valves, with rotation and torque imparted to the nut (or rod) by hand, by draught animals, with internal combustion engines, with electric motors, and anything else capable of exerting torque or tangential force on a lever arm.
Pneumatic and hydraulic rotary actuators are widely used with the object of generating rotation and torque for the operation of valves and various pivoting mechanisms, and come in a variety of constructions.
The simplest rotary actuators have one or more fluid-driven pistons arranged perpendicularly to the axis of an output shaft imparting torque and rotation through rack and pinion engagement with the shaft. These are commonly used to operate gate valves, but the lateral arrangement of their pistons makes them bulky.
Another type of rotary actuator features a chamber in the form of a partial annulus centered on the output shaft and a radial fin attached to the output shaft to divide the chamber in two parts. This type is also bulky.
A more elaborate kind of rotary actuator uses a piston in form of an annulus which engages a fixed housing and an output shaft by the means of helical splines or cam slots. This type of rotary actuator is more compact because the piston is coaxial with the output shaft, and is used in applications where the room available for an actuator is limited.